Any conventional vehicle is provided with a transmission system between the internal combustion engine and the driving wheels. Any conventional transmission system varies the driving force and speed of the wheels in accordance with widely variable driving conditions of the vehicle itself, to allow the engine to fully exert optimum performance. There are a variety of continuously variable transmissions systems, for example those which are disclosed in official Japanese Patent Laid-Open Publication Nos. 57-186656 (1982), 59-43259 (1984), 59-77159 (1984) and 61-233256 (1986). Any of these conventional, continuously variable transmission systems transmits the driving force through expanding or contracting the radius of rotation of a belt by selectively expanding or contracting the width of a channel formed between a stationary pulley member secured to a revolving shaft and a movable pulley member supported on the revolving shaft so that the movable pulley member can come into contact with and leave the stationary pulley shaft before eventually varying the belt ratio.
Any conventional clutch-pressure control system of a conventional continuously variable transmission executes feedback control over the objective (or in other words desired) clutch pressure using the actual clutch pressure when any of a hold, a start, and a special start mode of clutch control is activated. A controller unit calculates a clutch duty ratio which is used to drive a clutch-pressure control valve in order to minimize clutch pressure when the value of clutch pressure is maximum and vice versa in the event a pressure sensor goes out of order or a signal cable is either disconnected or short-circuited. As a result, the clutch is compulsorily held in an ON or an OFF condition, thus inhibiting the vehicle from starting to move or causing a sharp shock to be generated from the clutch engagement.
A primary object of the invention is to fully overcome the problems mentioned above by providing a novel method of controlling clutch pressure for use with a continuously variable transmission system which includes the following: a controller unit which executes control operation by feeding the actually detected clutch pressure back to the objective clutch pressure; and an oil-pressure circuit which controls clutch pressure in response to signals from the controller unit. The clutch-pressure control system according to the invention is to cause the controller unit to calculate a clutch duty ratio in reference to the objective clutch pressure and objective line pressure, and is to allow the controller unit to properly control clutch pressure in accordance with the calculated clutch duty ratio. Consequently, the clutch-pressure control system according to the invention is to properly control clutch pressure even when the pressure sensor malfunctions, when a signal cable is either disconnected or short-circuited, or when atmospheric temperature drops extremely, by generating a satisfactory control effect almost equivalent to that during a normal condition of use, thus securely preventing an unwanted failure which results in an inability to start movement of the vehicle or results in the occurrence of a sharp shock from the clutch. This offers a practical advantage for all drivers of vehicles.